1. Field of the Invention
The present invention relates to a process for producing an electrode for an electric double layer capacitor and to a process for producing an electric double layer capacitor employing such an electrode, in particular, to a process for producing an electrode for an electric double layer capacitor, wherein water is used as a forming auxiliary agent and kneading and pulverizing steps are omitted to improve production efficiency, and to a process for producing an electric double layer capacitor employing such an electrode.
2. Discussion of Background
In recent years, electric double layer capacitors, using an electric double layer formed at the interface between a polarized electrode and an electrolyte, particularly those having a coin shape, are increasingly demanded for power sources for memory backup. Meanwhile, for applications requiring large capacities such as power sources for electric automobile use, development of electric double layer capacitors of large capacity per a unit volume, low internal resistance, high energy density and high output density, is desired. Further, for the electric double layer capacitors for memory backup use, reduction of internal resistance is desired.
Here, FIG. 5 shows an example of vertical cross-sectional view of an electric double layer capacitor. Further, FIG. 6 shows a construction view of its element assembly.
In FIGS. 5 and 6, in a single cell 50 of the electric double layer capacitor, an element assembly 1 of column shape impregnated with an electrolytic solution (not shown) is accommodated in an outer casing 21 of cylindrical shape having a bottom portion 21a, and an opening 21b of the outer casing 21 is sealed with a sealing plate 31.
Here, the element assembly 1 is constituted by laminating a pair of electrode bodies 3A, 3B of long strip shape for forming electric double layers at the interfaces between the electrode bodies and the electric solution, and separators 5A, 5B disposed between the electrode bodies 3A, 3B. These electrode bodies 3A, 3B and the separators 5A, 5B are wound to form the element assembly 1. Here, one of the electrode bodies 3A, 3B constitutes a positive electrode, and the other one of them constitutes a negative electrode.
Further, the electrode bodies 3A, 3B have electric collector foils 7A, 7B respectively of long strip shape made of metal, and electrode layers 9A, 9B formed on surfaces of the electric collector foils 7A, 7B respectively. The electrode layers 9A, 9B are composed of a carbonaceous material mainly contributing to exhibit electrostatic capacitance, and a conductive material mainly functioning to increase conductivity, and the electrode layers 9A, 9B are each formed into a sheet shape of about at most 1 mm thick by employing a binder such as polytetrafluoroethylene. The electrode layers 9A, 9B are employed as they are bonded to the electric collector foils 7A, 7B.
As the material contributing to exhibit electrostatic capacitance, a carbonaceous material such as an activated carbon or a polyacene having a specific surface area of a few hundreds to about 3,000 m2/g, are mainly employed. As the conductive material for mainly increasing conductivity, a carbon black having a specific surface area of about 500 to 1,500 m2/g, is mainly employed.
As a method for producing sheet-shaped electrodes, an extrusion molding method has been known, in which a carbonaceous material, a binder and a liquid forming auxiliary agent are mixed, kneaded, and formed into a plate shape of a few millimeters thick by an extrusion method, and the product is rolled into a thin sheet shape (refer to JP-A-63-107011 and JP-A-11-283887).
Or else, a calendar forming method has been known, in which a carbonaceous material, a binder and a liquid forming auxiliary agent are mixed and kneaded, and thereafter, an obtained mixture of clay state or clumpy rubber state is preformed into a stick shape or pulverized into grain state or powder state, and the mixture is passed between two opposing rolls to be directly formed into a sheet shape (refer to JP-A-2001-307964, JP-A-2-235320, JP-A-2001-85280, JP-A-2001-35756 and Japanese Patent No. 3693254).
Among these forming methods, a calendar forming method is more excellent than an extrusion molding method in that it can mold the mixture directly into a thin sheet shape of at most about 1 mm thick. On the other hand, in the extrusion molding method, strong sheering force exerts to the mixture when it is extruded, which promotes unification of the mixture, while in the calendar forming method, sheering force exerted to the mixture at the time of forming is so small that homogenization of the mixture is scarcely promoted. To cope with this problem, in a case of forming the electrode sheet by the calendar forming method, it is considered to be necessary to sufficiently uniformly mixing the carbonaceous material, the binder and the forming auxiliary agent in advance.
The conductive material such as a carbon black is essential to reduce the resistance of an electric double layer capacitor to exhibit its performance, particularly high-output performance. As characteristics of a carbon black for efficiently imparting conductivity, 1) highly developed structure, 2) large specific surface area, 3) small primary particle size, 4) highly developed crystal structure, and 5) small amount of functional groups capturing π electron, are listed (source: Saishin Carbon Black Gijutsu Daizenshu; edited by Technical Information Institute Co., Ltd.). As a commercially available conductive carbon black, it has been known that Ketjenblack (manufactured by Ketjenblack International) has a characteristically large specific surface area and shows excellent conductive performance.
However, a conductive material having such a large specific surface area has a large surface energy, small grain size and small bulk density, and thus, such a material tends to agglomerate together, and it has been difficult to uniformly mix such a material with a material such as an activated carbon exhibiting static electric capacitance by a common mixing method. When such an insufficient dispersion occurs, the conductive material becomes to be easily peeled off from an electrode sheet, which may cause deterioration of durability of the electric double layer capacitor or deterioration of working environment, such being not preferred. To cope with this problem, in order to prevent such an insufficient dispersion of conductive material and to uniformly mix such a material, employment of wet-type kneading step using a liquid forming auxiliary agent (kneading auxiliary agent) has been considered to be necessary.
Further, in order to form kneaded product of clay state or rubber clumpy state by the calendar method, a step of pulverizing such a kneaded product to make its grain size uniform is required, which makes production process complicated. Further, as the kneading auxiliary agent, it has been desired to employ water from a viewpoint of minimizing environmental impact, but since a carbon black as a conductive material is hydrophobic, the carbon black tends to agglomerate at a time of kneading to cause insufficient dispersion.
In order to solve such problems, heretofore, methods of mixing a carbonaceous material, a conductive material and a binder as uniform as possible before forming them into a sheet shape, have been proposed.
For example, JP-A-2001-307964, JP-B-7-44127 and JP-A-2001-85280 propose methods of employing an organic solvent as a forming auxiliary agent. However, when such an organic solvent is employed, impact on environment increases or an explosion-proof type facility should be necessary in an organic solvent removal step in the process of producing electrode sheets, and thus, a method requiring minimum organic solvent has been desired.
Meanwhile, Japanese Patent No. 3028560 and JP-A-2003-225547 propose a dry type forming method not employing an auxiliary agent. However, in such a method, since an activated carbon, a carbon black and polytetrafluoroethylene have significantly different physical properties, in a case where a rotating blade or a bar shaped tool is employed, the carbon black significantly tends to be suspended and the carbon black hardly be mixed into portions where mixing or compression occurs, and thus, it has not been possible to produce a long sheet having uniform characteristic and sufficient strength.
Further, for the forming step, methods of mixing and kneading a carbonaceous material such as an activated carbon, a conductive material such as a carbon black, a binder such as polytetrafluoroethylene and a forming auxiliary agent in advance to carry out forming, have been proposed (refer to JP-A-2001-35756 and Japanese Patent No. 3693254). However, since this method uses a kneading machine such as a kneader and the kneaded product becomes to be in a state of clumpy body, a step of pulverizing such clumpy bodies becomes required to deteriorate productivity or causes mixing of impurities from production equipment. Further, if too large sheering force is applied to such clumpy bodies by a kneading machine such as a kneader, PTFE as a binder is excessively fibrillated, which makes the sheet brittle and which may cause significant loss.